The present invention relates to a method of marking and of identifying objects which are normally non-conductors of electricity and marked by conducting elements. It is intended more particularly, by way of example, for recognizing the authenticity of documents thus previously marked and coded.
At present it is known to detect the presence of a metallic body which is a conductor of electricity in a mass or a region which normally only includes non-conducting elements. For this, a generator of high frequency electromagnetic waves is used, normally having a certain impedance. When a conducting mass, metallic for example, enters the field of emission, the eddy currents generated in the mass absorb some of the energy emitted by the generator and give rise to the emission of a reflected energy. The detection of such a response signal, or of the modification in the impedance of the system, reveals the presence of a conducting mass. Such devices are used, for example, for the detection of mines hidden in the ground, or for safety checks at airports. In all these cases, where the aim is purely qualitative, no effort is made to measure the disturbances recorded.